The present invention relates to a process for separating mixtures which comprise sulfur hexafluoride (SF6) and nitrogen (N2) and which originate from gas-insulated lines (xe2x80x9cGILxe2x80x9d).
Mixtures of sulfur hexafluoride and nitrogen are used as insulating filler gas for underground cables, see German Utility Model 297 20 507.2. Usually, these mixtures comprise from 5 to 50% by volume of sulfur hexafluoride, and the remainder to 100% by volume nitrogen.
In the context of servicing the lines, or in the event of defects in the lines, it is desirable to be able to separate the gas mixture, in particular for the purpose of reusing the SF6. The resulting SF6 then occupies a very small volume, which is a significant advantage during transport, design of line cross sections, etc.
Li et al., EP 853,970 and U.S. Pat. No. 5,785,741 (=EP 754,487) describe a process for separating gas mixtures, which arise during the manufacture of semiconductors. Gas mixtures of this type may contain perfluoro compounds. The separation of the gas mixtures takes place at membranes. Anumakonda et al., U.S. Pat. No. 5,843,208 describes a process for recovering SF6 from gas mixtures using membranes at a pressure of at most 6.2 bar.
Despite the efforts of the prior art, there has remained a need for improved techniques for separating mixtures of SF6 and N2.
It is an object of the present invention to provide a new process for separating the abovementioned gas mixtures.
Another object of the invention is to provide a separation process which makes it possible to isolate the SF6 from the mixtures so that it can be reintroduced into the gas-insulated line or reused in a closed product cycle.
A further object is the provision of a suitable apparatus for separating such mixtures.
The process of the invention provides that SF6/N2 mixtures originating from gas-insulated lines are separated by means of membranes which are able to separate sulfur hexafluoride. The process of the invention can be carried out, for example, during the servicing of gas-insulated high-voltage lines, or in the event of faults, or if there is evidence that the gas in the line requires regeneration. The separated sulfur hexafluoride can be recycled into the gas-insulated line. Depending on the concentration desired, nitrogen is also introduced into the line.
Another possibile use of the process of the invention is to process or work-up mixtures of SF6 and N2 (with or without other impurities) which are found in a gas-insulated line, when the use of the gas-insulated line is terminated and the line is to be scrapped. The SF6 can be recovered from the mixture and supplied for reprocessing.
The SF6 content typically is in the range from 5 to 50% by volume. However, the process of the invention can also be used to separate gas mixtures having a higher SF6 content.
Organic, asymmetric membranes are preferred. Rubbery membranes are known which separate based on the solubility of the permeate. Other membranes separate based on the diffusion property of the permeate; these are non-rubbery membranes, or rather crystalline membranes (xe2x80x9cglassy membranesxe2x80x9d). This latter type of membrane is especially preferred in the process of the invention.
The membranes can be constructed in a known manner, for example as a membrane made of one or more bundles of hollow-fibers. The membrane can be produced from known materials. Examples of suitable materials include polyimides, polycarbonates, polyesters, polyestercarbonates, polysulfones, polyethersulfones, polyamides, polyphenylene oxides and polyolefins. Preferably, the polymer material is comprised of polyesters, polycarbonates or polyestercarbonates. Membranes made of polycarbonates which are derived from a bisphenol in which at least 25% of the bisphenol units in the polymer chain are tetrahalogenated with chlorine or bromine are especially suitable. Particularly preferred membranes have a polymeric matrix which has two porous surfaces and one layer which makes it possible to separate the sulfur hexafluoride from the other gas constituents. Membranes of this type are described in Sanders et al., U.S. Pat. No. 4,838,904 (=EP 340,262). If additional impurities such as SO2F2, SO2, or the like are present in the gas mixture, a purification can be carried out in advance, such as washing with water or alkali metal hydroxide solution or treatment with adsorbers.
The separating membranes may advantageously be arranged in a plurality of separating stages. Each membrane stage can consist of a plurality of membrane cartridges arranged in parallel.
The pressure on the inlet side of the membrane or membranes is customarily higher than the ambient pressure. For example, the gas mixture to be separated can be fed to the separating membrane at a pressure of up to 13 bar. Preferably, the membrane feed pressure is from 10 to 12 bar. If a plurality of membranes are provided, a compressor can be disposed upstream of each membrane. The temperature of the gas mixture is advantageously in the range from 10xc2x0 C. to 40xc2x0 C.
If two membrane separation stages are provided, the gas streams may advantageously be conducted in the following manner: the mixture to be separatedxe2x80x94for example a mixture of sulfur hexafluoride and nitrogen containing 20% by volume of SF6 from gas-insulated high-voltage linesxe2x80x94is fed to the first membrane. Since the membrane preferentially allows nitrogen to pass, a permeate having a high nitrogen content and a low sulfur hexafluoride content is obtained. The permeate is discharged into the environment. The retentate of the first membrane, which already has an elevated SF6 concentration, is fed to a further membrane, i.e., a second membrane separation stage. The permeate resulting from this second membrane is recycled back into the feed stream of the first membrane. The retentate from the second membrane is sulfur hexafluoride containing only low amounts of nitrogen. After liquefaction by a compressor, it can be recycled immediately into the gas-insulated high-voltage line, or it can be stored temporarily and reused in other ways.
The number and arrangement of the membrane cartridges depends on the desired degree of purity and on whether a gas having high or low SF6 content is to be treated. An even better membrane separation effect can be achieved if three membrane stages are employed. Preferably, the three membranes are connected as follows: the SF6/N2 gas mixture is fed to the first membrane stage as feed stream. The retentate of the first membrane stage is fed to a second membrane stage as feed stream. The retentate of this second stage is highly enriched SF6 and is reprocessable. The permeate of the first membrane stage is used as the feed stream to the third membrane stage. The permeate of this third stage is N2, virtually free of SF6, and can be safely discharged into the environment. The permeate of the second membrane stage and the retentate of the third membrane stage are introduced into the feed stream to the first membrane stage.
It has been found that even one or two membrane stages are sufficient to be able to obtain a sufficiently enriched, purified sulfur hexafluoride and a nitrogen gas having acceptably low amounts of sulfur hexafluoride. A downstream adsorption stage is not needed and therefore is not included in prefered embodiments.
The process of the invention is characterized by excellent separation of the SF6/N2 mixture from underground cables. The purified nitrogen or purified air can be safely discharged into the environment. The emission of SF6 into the environment is greatly decreased. The recovered sulfur hexafluoride can be re-introduced immediately back into the gas-insulated high-voltage line. However, other operations can also be performed, for example admixing nitrogen, in order to obtain a gas mixture containing desired proportions of the gases.
The invention also relates to a system comprising a gas-insulated, SF6/N2-filled high-voltage line, a membrane separator installation and connection lines between the gas-insulated high-voltage line and the membrane separator installation. The membrane separator installation may comprise one, two, three or more membrane separation stages having membranes which are preferentially permeable to nitrogen. The foregoing numbers apply to the number of membrane stages. Upstream of the first membrane stage, and preferably upstream of each other membrane stage, there is a compressor. A preferred separator installation has at least two membrane separation stages. It further comprises a connection line for the gas mixture to be separated, which connection line is connected between the gas-insulated high-voltage line and the inlet into the first membrane separation stage. If there are two separation stages, the installation comprises a second connection line between the first and second membrane separation stages, which line is provided for introducing retentate (enriched in SF6) from the first membrane separation stage into the second membrane separation stage. There is also a collection line for retentate from the second membrane separation stage, through which retentate having a high SF6 content can be collected. This collection line connects the membrane separation plant either back to the gas-insulated high-voltage line (connection line for recycling the SF6) or to a tank for temporary storage. In addition, the separator installation has a return line for feeding the permeate of the second membrane stage back into the feed stream of the first membrane stage. Pumps (for example vacuum pumps) and compressors for withdrawing and feeding in the gas mixture or SF6 are provided between gas-insulated line and the membrane separation installation. If desired, other treatment devices can be connected intermediately (compressor, gas mixture for N2 admixture etc.). The permeate of the first membrane stage is largely free of SF6 and can be discharged into the environment.